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2006 Progress Report: Electrocatalysis for Environmentally Friendly Energy Production Systems
EPA Grant Number: R831495Title: Electrocatalysis for Environmentally Friendly Energy Production Systems
Investigators: Pfefferle, Lisa , Schwartz, William R.
Current Investigators: Pfefferle, Lisa , McEnally, Charles
Institution: Yale University
EPA Project Officer: Bauer, Diana
Project Period: December 1, 2003 through November 30, 2006 (Extended to November 30, 2007)
Project Period Covered by this Report: December 1, 2005 through November 30, 2006
Project Amount: $375,000
RFA: Technology for a Sustainable Environment (2003)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:We are investigating three primary issues with respect to the catalytic combustion of methane over Pd based catalysts: catalyst deactivation over time in a low temperature regime, bi-metallic based catalytic activity in comparison to Pd based catalytic activity, and electrochemical promotion of oxygen transport to Pd reaction sites through a yttria-stabilized zirconia (YSZ) membrane. This progress report summarizes our current findings and our planned future work.
Progress Summary:Catalyst Deactivation
The overall reaction for the complete oxidation of methane is:
CH4 + 2O2 → CO2 + 2H2O, ∆H298 = -192 kcal/mole
We hypothesize that Pd based catalysts deactivate over time during the methane oxidation reaction at temperatures below 450°C as a consequence of hydroxyls accumulating on the catalyst surface, which blocks active catalytic sites. We also hypothesize that for Pd catalysts that are supported by metal oxides, catalyst deactivation correlates with oxygen mobility between the Pd and metal oxide. Catalyst supports with higher oxygen mobility are expected to have lower deactivation rates, since hydroxyls can more easily move away from active catalytic sites. If this explanation is correct, electrochemical oxygen pumping should also improve catalytic activity from hydroxyl displacement by oxygen.
Our strategy to investigate these hypotheses is to identify hydroxyl peaks on palladium and then observe any correlations between catalytic activity, hydroxyl surface coverage of Pd, and oxygen mobility of metal oxide supports.
To identify hydroxyl peaks on Pd sites, we prepared multiple Pd based catalysts with various support materials (to increase Pd dispersion) and then used Fourier transform infrared (FTIR) spectroscopy to identify common peaks in each of the catalyst samples that can be attributed to the common element Pd. Materials studied so far include Pd/MCM41, Pd/TiO2, and Pd/Al2O3. To the extent that pure metal oxides show different hydroxyl peaks, any remaining OH peaks in the Pd based samples must be attributable to OH groups adsorbed to Pd sites.
Prior studies have looked at the catalyzed methane oxidation reaction and the correlation between water concentration and catalyst deactivation. It has been determined that a first order relationship exists between water concentration and catalyst activity when water is present in the reaction system. To gain insight into the mechanism for catalyst deactivation during reaction, this study looks at the catalytic reaction in the absence of an external water source.
OH Analysis Using FTIR
Hydroxyl vibrational energy is in the infrared region in the range from about 3400–3800 cm-1 and is easily detectable using FTIR spectroscopy. In order to detect and analyze hydroxyls taking part in a catalytic reaction, other environmental hydroxyls must be removed. These include hydroxyls from water vapor that exists in reactant gas flows and atmospheric water vapor in the infrared (IR) beam path.
Water vapor was removed from the reactant gas flow by a liquid nitrogen cold trap. It was removed from the IR beam path by flowing nitrogen gas into the interior of the FTIR apparatus at a constant flow rate.
Pd/Al2O3
Figure 1 illustrates a typical FTIR spectra obtained for 5 % Pd/Al2O3 at 225°C.
Figure 1. Typical Spectra Obtained During In-Situ Methane Oxidation Reaction
FTIR spectra were taken in series lasting 40 minutes. During the first approximately half of each series, reactant gases flow into the FTIR sample chamber. During the second half of the series, methane flow was stopped.
To view the relationship between CO2, CH4, and hydroxyl peaks over time at various temperatures, Figures 2, 3, and 4 show normalized peak heights for selected wavenumbers as a function of time. From the blue methane normalized absorbance values, it is readily apparent when methane flow was introduced and when it was switched off.
![Figure 2. CO[2], CH[4], and OH Normalized Peak Heights over Time for Pd/Al[2]O[3] at 225[C]](https://webarchive.library.unt.edu/eot2008/20081107222928im_/http://es.epa.gov/ncer/progress/images/R831495_06_002.gif)
Figure 2. CO2, CH4, and OH Normalized Peak Heights over Time for Pd/Al2O3 at 225°C
![Figure 3. CO[2], CH[4], and OH Normalized Peak Heights over Time for Pd/Al[2]O[3] at 325[C]](https://webarchive.library.unt.edu/eot2008/20081107222928im_/http://es.epa.gov/ncer/progress/images/R831495_06_003.gif)
Figure 3. CO2, CH4, and OH Normalized Peak Heights over Time for Pd/Al2O3 at 325°C
![Figure 4. CO[2], CH[4], and OH Normalized Peak Heights over Time for Pd/Al[2]O[3] at 450[C]](https://webarchive.library.unt.edu/eot2008/20081107222928im_/http://es.epa.gov/ncer/progress/images/R831495_06_004.gif)
Figure 4. CO2, CH4, and OH Normalized Peak Heights over Time for Pd/Al2O3 at 450°C
Figures 2, 3, and 4 demonstrate that catalytic deactivation, as reflected by the green CO2 trace, occurs at the tested temperatures below 450°C. Notice in Figures 2 and 3 that the hydroxyl and CO2 peaks have an inverse relationship during catalyst deactivation. If some factor in the catalytic reaction other than the increase in hydroxyls were the cause of the catalyst deactivation, then we would expect both CO2 and OH concentrations to decrease, since they are both products from the methane oxidation reaction. Therefore, the inverse relationship between CO2 and OH suggests that there is a cause and effect relationship, whereby the increased hydroxyl coverage causes catalytic activity to decrease. Also note the strikingly different hydroxyl coverage at different temperatures after methane flow is terminated. At 225°C, there is practically no hydroxyl desorption from the surface, while at 450°C, desorption is significant after methane flow is stopped. This provides further evidence that catalytic deactivation is associated with hydroxyl surface coverage.
While Figures 2, 3, and 4 address the relationship between CO2, methane, and hydroxyls, the interrelationship between the different hydroxyl peaks is also of interest. Surface hydroxyls are adsorbed to Pd and Al, and a relationship between the hydroxyl peaks may help to establish the identity of the Pd-OH peak and the mechanism for catalyst deactivation.
Two-dimensional (2D) correlation analysis has been shown to be a useful technique for determining relationships between spectroscopic peaks after a perturbation, such as the start or cessation of a chemical reaction. It can distinguish peak changes over time from noise and can also determine if related peaks are moving together synchronously or if one peak is moving first or faster than another peak (Noda, 1993).
2D orrelation analysis was used in this study to gain additional insight into the relationship between the four distinguishable hydroxyl peaks shown in Figure 1. Figures 5 and 6 illustrate the synchronous and asynchronous 2D correlations for the IR hydroxyl vibrational region of interest between 3465 and 3760 cm-1.
Figure 5. Peaks of Interest at 3508, 3564, 3690, and 3734 Show Positive Cross Peaks
Figure 6. Peak Growth at 3508, 3564 Occurs at a Faster Rate than the Peaks at 3690 and 3734
The synchronous plot in Figure 5 indicates that the hydroxyl peaks of interest at wavenumbers 3508, 3564, 3690, and 3734 are growing together. The positive cross peaks indicate that both peaks are moving in the same direction.
Given the positive cross peaks in Figure 5, the asynchronous plot in Figure 6 indicates that the peaks at 3508 and 3564 are growing first or faster comparison to the peaks at 3690 and 3734. This is determined from the positive values at points (3508, 3690), (3564, 3690), (3508, 3734), and (3564, 3734).
Pd/TiO2
Figure 7 illustrates typical FTIR spectra for Pd/TiO2 at 400°C.
Figure 7. Typical Spectra Obtained During In-Situ Methane Oxidation Reaction
FTIR spectra were taken in series for 70 minutes. During the first 45 minutes of the series, reactant gases flow into the FTIR sample chamber. During the remaining 25 minutes of the series, methane flow was stopped.
In comparison to the Pd/Al2O3 catalyst, which has a broad hydroxyl peak region, the Pd/TiO2 shows a relatively narrow peak at 3664 attributable to Ti-OH. We also note that there is relatively little catalytic deactivation, as measured by the change in size of the CO2 peak during methane combustion. The relatively stable methane combustion may be attributable to the greater oxygen mobility on Pd/TiO2, whereby hydroxyls formed at the catalytically active PdO sites are freer to desorb from those sites. Surface oxygen atoms can then migrate and bind to Pd at vacant sites.
Bi-Metallic Based Catalytic Activity
Methane catalytic combustion activity was compared over 5 % Pd/Al2O3 and 5 % Pd-Pt/Al2O3 catalysts. With respect to catalytic activity, it was observed that the Pd-Pt based catalyst is considerably more stable and exhibits less deactivation than the Pd based catalyst.
The activation energy for hydroxyl desorption is less for platinum than for palladium (Johansson and Rosen, 2003), and a suggested surface transitional step whereby hydroxyls combine to form water is: *OH + *OH → H2O + O* (Johannson and Rosen, 2003; Fridell, et al., 1991). From a thermodynamic perspective, this step is more favorable on a Pd-Pt catalyst than on a Pd catalyst, and may contribute to the improved catalytic performance observed on the Pd-Pt based catalyst.
We have submitted a paper for publication that addresses the greater catalytic stability of Pd-Pt catalysts in comparison to Pd based catalysts.
Electrochemical Promotion of Oxygen Transport Through YSZ
Our objective with respect to an electrochemical promotion is to develop a Pd based catalytic reactor for the combustion of methane, in which a YSZ membrane will allow oxygen to pass from the atmosphere into the reactor, while CO2 product is sequestered within the reactor. Initial experiments with YSZ focus on applying a potential to electrochemically improve methane conversion.
The initial experiments were configured with methane, oxygen, and helium flowing into a Pyrex reactor vessel containing a YSZ pellet with a thin layer of Pd on one side of the pellet. Gold electrodes were also attached to the pellet in order to generate an electric field through the pellet. The pellet was held in place by gold wires attached to the gold electrodes, and the gold wires ex-tended to a potentiostat voltage source. Product gases flowed from the reactor to a gas chromatography instrument, which measured the concentration of methane and CO2.
Initial results of applying a potential on a Pd/YSZ catalyst are shown in Figure 8. Clearly the addition of excess oxygen to the reactant flow has a dramatic effect on conversion levels. Electrochemical promotion also induced a positive change in conversion levels.
Figure 8. Gas Chromatography Analysis of Methane Conversion during Methane Oxidation Reaction
The data in Figure 8 were obtained by first flowing a stoichiometric ratio of methane and oxygen in helium carrier gas overnight so that a steady state conversion level was achieved. A 1 volt potential was then applied through the Pd/YSZ sample until conversion reached a new steady state. The voltage was then discontinued. Subsequently, excess oxygen was introduced into the reactant flow and after conversion leveled off, a 1 volt potential was again applied until conversion stabilized. Finally, the potential was removed while excess oxygen continued to flow.
Future Activities:Identification of Hydroxyl Vibrational Energy Peaks on Pd
We have already looked at the hydroxyl vibrational energy peaks on Pd catalysts supported by Al2O3, TiO2, and MCM41. Additional FTIR in-situ studies are planned with CaO, NiO, CeO2, and MgO supports. These metal oxides were chosen for their high surface area, good catalytic performance, and varying oxygen mobility. From this sampling, we hope to distinguish hydroxyl peaks on Pd from hydroxyl peaks on the metal oxide support.
We are also in the process of fabricating a high surface area Pd nanostructure. The plan is to create sufficient Pd surface area so that hydroxyl peaks might be identified without the need for a metal oxide support.
Electrochemical Promotion of Oxygen Transport
In order to conduct electrochemical promotion and conversion tests on Pd/YSZ samples at temperatures below 450°C, methane conversion must be improved. We are working on two methods to accomplish this. First, we are redesigning the methane oxidation reactor to improve reactant gas flow over the Pd/YSZ catalyst surface. Second, we are working on new methods to improve Pd dispersion on a YSZ pellet.
Once sufficient conversion levels are achieved, we will be able to test whether electrochemical promotion can eliminate or reduce catalytic deactivation due to hydroxyl formation.
We are also investigating the sensitivity of methane conversion rates to oxygen concentration.
Finally, we are designing a YSZ based reactor, where the reactor walls serve as the membrane which oxygen must pass through in order for methane combustion to occur. We will determine whether electrochemical promotion can be used to transport oxygen at a sufficient rate to generate significant methane conversion levels.
References:
Noda I. Generalized two-dimensional correlation method applicable to infrared, Raman, and other types of spectroscopy. Applied Spectroscopy 1993;47(9):1329-1336.
Johansson A, Rosen A. A study of the apparent desorption energy for OH in the water formation reaction on a palladium catalyst. Catalysis Letters 2003;91(1-2).
Fridell E, Hellsing B, et al. Hydroxyl desorption from platinum in the catalytic formation and decomposition of water. J Vac Sci Technol A 1991;9(4):2322-2325.
Journal Articles:No journal articles submitted with this report: View all 5 publications for this project
Supplemental Keywords:, INTERNATIONAL COOPERATION, Sustainable Industry/Business, Scientific Discipline, RFA, POLLUTION PREVENTION, Technology for Sustainable Environment, Sustainable Environment, Chemical Engineering, Energy, Chemicals Management, Environmental Chemistry, Economics and Business, energy conservation, catalysts, green chemistry, zero emissions combustors, oxidation reactions, environmentally benign catalysts, catalysis, energy efficiency
Progress and Final Reports:
Original Abstract
Final Report